If you've stuck with me this far, you're probably wondering what on Earth could possibly be left to talk about. You've done a bazillion reps, you've got a pretty good handle on all the weird variations. What else is there?
Take the "decision" out of it.
You might think you've done so many repetitions, you could do your ukemi in your sleep. True, but there's one thing I still see present in most students who have come this far that gets in their way: they've made the "how" automatic, but not necessarily the "when."
Doing ukemi while training for ukemi is one thing; falling when you don't expect it, or intend it is quite another. So take your falls during class. I'm not saying fall down for everything anyone does to you; the technique has got to be there. But I see far, far too many people in class resisting the fall altogether. They're choosing whether or not to fall, and that will get them hurt, believe it or not.
Take your falls, take your falls. Even when, for example, the instructor is using you for a demonstration and you both assume he's only planning on "loading it up" and doesn't intend to "finish" it and throw you, and you loose your balance to the point that you're on the edge of the cliff, FALL. Granted, the instructor will think you're jumping for him. Stand up, apologize, move on. But as far as I'm concerned, always, always err on the side of falling.
For what it's worth, I'll tell you right now, I don't jump for people. I'm not doing you any favors by doing the work for you. Of course, I'll "broaden the window" so to speak with newer students, but if they don't have it, I won't fall.
The odd thing is, when I do fall, they sometimes accuse me of jumping, mostly, I assume, because they know their own technique was not that good. Here's the thing, though: Just because you do a mediocre technique, doesn't mean I'm going to do a mediocre fall. I'm the one hitting the ground, so I'm going to do it in the best, most efficient and safest way I know how.
Ultimately, I can't be in the habit of finding myself on the edge of the cliff and having to take a millisecond to decide, should I fall or not?
Some years ago, I was walking at night, downtown. I was chatting with some friends as I walked and wasn't paying attention and walked right off the curb. My ankle buckled. I didn't resist in the slightest, but melted with it, tucked and rolled back into a standing position. I kept walking and talking, and my ankle was fine. I got quite a few queer looks, however, but I walked away from it. Any hesitation on my part, though (should I roll, try to keep my balance, etc.) would probably have forced me to tweak my ankle or worse.
I've heard a number of similar stories over the years, and had a few other similar experiences myself. Chances are, you will, too some day.
But this is a critical step in any aspect of aikido, isn't it? Taking the conscious mind out of it. Flowing, going with the energy, accepting what is, being what is. And in my mind, that seems to be the defining difference between the good ukemi artists and the great ones.
Amazing ukemi, Step 1